Process Equipment and Plant Design
When one of the fluids is highly corrosive and has fouling tendency, it should

Preferably flow inside the tube for its easier internal cleaning
Perferably flow outside the tube
Flow outside the tube, when the flow is counter-current and inside the tube when the flow is co-current
Flow at a very slow velocity

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Process Equipment and Plant Design
For turbulent flow (NRe > 2100) of low viscosity fluid (μ > 20cp) in steel pipes, the optimum inside pipe diameter is given by(where, Q = fluid flow rate, ft³/sec.ρ = fluid density, lb/ft³ μ = fluid viscosity, centipoise Di = optimum inside pipe diameter, inches)

Di, opt = 3.9 Q0.45 . μ0.95
Di, opt = 3 Q0.36 . μ0.88
Di, opt = 4.7 Q0.36 . μ3.2ρ0.13
Di, opt = 3.9 Q0.45 ρ0.13

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